1. Field of the Invention
The present invention relates to a method for calibrating a navigation system, of the type which generates localization data in a first coordinate system, in relation to image data, the image data being produced by a magnetic resonance apparatus in a second coordinate system. In addition, the invention relates to apparatuses for conducting the method.
2. Description of the Prior Art
The position of the coordinate system in a diagnostic magnetic resonance apparatus is determined by the basic field magnets, including shim elements and the gradient coil system. It can be indicated with a precision up to a few millimeters in relation to the outer covering of the apparatus. A patient is, for example, placed in the imaging volume using a laser light sight attached to the covering of the magnetic resonance apparatus, the sight having a known distance to a midpoint--defined by the housing, of the imaging volume. For many magnetic resonance measurements, the precision thus achieved is sufficient. In certain cases, e.g. if the representation of symmetries in the body in the magnetic resonance image data set is concerned, a scan known as a scout scan is carried out before the actual determination of the image data, wherein sectional images in axial, coronal and sagittal orientations produced precisely through the center of the magnetic resonance coordinate system. On the basis of these three sectional images, the position of the patient in the magnetic resonance apparatus is corrected.
Primarily in neurosurgery, navigation systems are increasingly used in order to increase surgical precision, and in order to improve degree of radicality in an operation on tumors. In general, in neuronavigation particular position points in the operating field are projected on preoperative sectional image data. In this way, the surgeon can determine the position of deeper-lying structures or lesions in the operating field by means of the sectional image data, and thus can minimize the access path.
In neuronavigation, the image data coordinate system must first be brought to coincide with the patient coordinate system using the navigation system, i.e., the navigation system must be calibrated. Conventionally, for this purpose markers, visible in the sectional image, are attached to various points on the surface of the head before image data are recorded. After the patient has then been fixed for the operation, the markers imaged in the image data set can be marked with a mouse or with cross hairs. The markers are likewise located using a display instrument (pointer) of the navigation system. In this way, an image data computer can allocate the various marker points, and can bring the coordinate systems into coincidence. This method not only is time-consuming, but also conceals various sources of error. For example, the markers on the skin may move after being affixed to the head. Imprecisions also result if the marker points are not marked precisely with the cross-hairs. Another source of error can occur if the tip of the display instrument of the navigation system cannot meet the marker center precisely.
In order to allow displacements of the brain anatomy after the opening of the skull, or due to the use of brain spatulas, to be taken into account, in recent practice the 3D magnetic resonance sectional image data for the navigation system are newly acquired during the operation, using a magnetic resonance apparatus used intraoperatively; i.e., the basis for the navigation system is no longer a 3D image data set produced preoperatively, but rather a current image data set recorded intraoperatively after shifts or displacements of the brain anatomy.
For various interventions, such as freehand biopsies or brain biopsies without stereotactic frames, as well as for many interventional techniques with magnetic resonance sectional image guiding (MR guiding), it is useful to integrate the navigation system directly into the magnetic resonance apparatus. In this way, it is possible, for example, to set a particular position of the sectional image interactively at the patient, or, given coupling of the display instrument of the navigation system with the biopsy needle, to ensure that the image slice constantly follows the position of the biopsy needle. In this case, biopsies can be carried out extremely rapidly and with high precision. Similar to the way described above, it is necessary to make the coordinate system of the magnetic resonance apparatus (which in this case is identical to the patient coordinate system) known to the navigation system; i.e., the navigation system must be calibrated.
Conventionally, for the calibration of the navigation system, points on a surface of the magnetic resonance apparatus have been approached with the tip of the display instrument of the navigation system, the position of these points relative to the magnetic midpoint (which is at the same time the center of the patient coordinate system or of the image data coordinate system) being known. Since the distance of the housing points to the magnetic midpoint always has a tolerance that cannot be disregarded, and in addition the magnetic midpoint also depends on the adjustment of the gradient system, this type of calibration is subject to considerable error.
From German OS 43 25 206 and German PS 38 31 278, orientation arrangements are known that permit measurement of distances, e.g. of anatomical details, in a slice of a subject under examination in a corresponding tomogram, using markers.